A representative matrix-addressed flat panel display in the prior art is described in U.S. Pat. No. 4,857,799 which is incorporated herein by reference. The display described there includes a transparent face plate or structure mounted over and spaced from a backing plate or structure so as to define an interior chamber. The transparent face plate carries on its internal surface a thin coating or film of electrically conductive transparent material, such as indium tin oxide, which serves as an accelerator plate. Individual phosphor-coated stripes or dots are provided over this latter film. The internal surface of the backing plate supports a matrix array of field emission cathodes in confronting relationship with the face plate and suitable address means for energizing selected ones of the field emission cathodes, thereby causing the energized cathodes to bombard the phosphor-coated stripes on the face plate with electrons which, in turn, results in the emission of visible light. It is this light that is viewed by the observer through the face plate of the flat panel display.
While the matrix-addressed flat panel display disclosed in U.S. Pat. No. 4,857,799 is generally satisfactory for its intended purpose, there are relatively large light losses due to the radiation toward the baseplate. Specifically, as is evident in U.S. Pat. No. 4,857,799 , some of the light is lost through the phosphor stripes and all of the light directed toward the backing plate is lost to the observer. As a result, in order to provide sufficient light to the observer, the field emission cathodes must be driven at relatively high voltages so as to provide sufficiently high energy electron beams.
As will be seen hereinafter, the present invention eliminates the draw back described immediately above by providing a matrix-addressed flat panel display which is viewed, by the observer through a transparent backing plate rather than through the face plate. As a result, the electron bombarded side of the phosphor coating can be viewed directly and, as will be seen, some of the light emitted behind the phosphor coating can be redirected toward the observer. In that way, the field emission cathodes or other such light generating means can be driven at lower voltages than would be otherwise required for providing the desired level of viewing light.
It should be noted at the outset that there are other types of display devices which utilize transparent face plates for the purpose of viewing displays. One such device is a liquid crystal display (LCD) in which a liquid crystal material is disposed between what may be characterized as a face plate and what may be characterized as a base plate. The face plate either carries a mirrored surface or its own independent source of light. Both the base plate, which is transparent, and the face plate carry a matrix-array of electrically conductive leads which are selectively energized in a way that selectively polarizes the liquid crystal material in order to allow the viewer to selectively view segments or pixels of the face plate in order to provide the desired display. Obviously, those electrical leads must either be constructed of transparent material or, if opaque, they must be sufficiently thin to be virtually invisible. The present state of the art is capable of providing both types of electrically conductive leads.
A second type of device that utilizes what may be characterized as a transparent cover plate for viewing purposes is a vacuum fluorescent display (VFD). Such a device is typically found on, for example, a microwave oven or other such appliance. Its transparent cover plate is spaced from and in confronting relationship with a phosphor screen which has a series of positively pulsing eight-segment fluorescent characters. A hot filament cathode wire and a suitable grid are positioned between the phosphor support plate and transparent cover plate for providing a continuous supply of electrons which bombard selected segments of the eight-segment displays for illuminating the latter.
While the VFD device utilizes what may be characterized as a transparent cover plate and relies on electron bombardment to energize its eightsegment fluorescent displays, the device does not require matrix-addressing of individual cathodes or use the cover plate in the electronic operation of the device. Rather, a single hot filament cathode wire is sufficient to provide a continuous supply of electrons. As a result, there is no real concern for obstructing the view of the observer through the transparent cover plate. On the other hand, the LCD device described above does, indeed, utilize a matrix-array of electrically conductive leads located on one side of its liquid crystal material. However, the matrix-addressing of those leads is not associated with the selective energization of individual field emission cathodes. The LCD device merely acts as a series of selective light valves which turn on and off by means of the addressing arrangements.